CLONING AND MOLECULAR CHARACTERIZATION OF A TELOMERIC SEQUENCE FROM A TEMPERATURE-INDUCED BALBIANI RING

In C. t. thummi a Balbiani ring-like structure is induced at the right end of chromosome III by heat shock. Telomere-specific clones were obtained by microcloning this chromosome region. One of the recombinant clones, .lambda. Cth5, which had an insert of 176 base pairs (bp), hybridized in situ to all the chromosome ends of C. t. thummi except one on the acrocentric chromosome IV. Homologous sequences were also present at 2 interstitial sites on chromosome II. Southern analysis of genomic DNA indicated that the 176-bp insert represents a basic repeating unit clustered in tandem arrays. Around 1000 copies of this sequence were located in each telomere. The repeats were homogeneous in size but showed some sequence divergence, which seemed to increase when different telomeres were compared. At the nucleotide sequence level the most noticeable feature was the presence of tracts of adenine alternating with tracts of thymine. In situ hybridization to chromosomal RNA showed that the 176-bp repeat was complementary to the RNA transcribed at the T-BRIII region, and occasionally, to the RNA transcribed at some of the other telomeres during heat shock. Northern blot analysis of total RNA showed that a transcript larger than 10 kilobase pairs (kb) and complementary to the telomeric insert was present in heat-shock as well as in untreated cells. At most only a very weak RNA hybridization was seen at the telomeres of untreated animals.